1. Field of the Invention
This invention relates to the automated control of liquid discharge from a control structure or storage reservoir, and more particularly to the controlled release of stormwater to a conveyance system from a stormwater detention reservoir or system of reservoirs by automatic adjustment of the release rate from the reservoir in order to support a variety. of water management objectives.
2. Discussion of the Problem Solved
Stormwater is water generated by rainfall and is often collected and routed into storm water management facilities to prevent downstream flooding, erosion, sedimentation and water quality degradation. Uncontrolled stormwater runoff from development, forest and agricultural activities can cause flooding, channel erosion, sedimentation and degradation of wildlife habitat and water quality.
Many urban and developed areas require stormwater management, the objectives of which include runoff rate control, erosion and sedimentation control, as well as water quality improvement. Accomplishment of these objectives is often attempted with stormwater detention reservoirs and water quality improvement structures. The term "detention reservoir," used herein, refers to a facility or structure capable of detaining, storing or withholding surface water or other liquids. This includes ponds, lagoons, below ground pipes, vaults, tanks, ditches, wetlands and tidal marshes, as well as water controlled by dams, dikes, weirs or risers.
Development can drastically change the hydrology of a site. Roads, driveways, sidewalks, roofs and lawns cause greater volumes of stormwater runoff at higher rates than under natural conditions. To control peak runoff rates from developed areas, stormwater is typically collected and routed to a detention reservoir where the stormwater is stored and released to the downstream system at a designed rate. The design release rate is often determined by the capacity of the downstream conveyance system and is frequently limited to a designated proportion of the predeveloped runoff rate. Exceeding the capacity of the downstream conveyance system can create flooding, erosion and sedimentation.
The release rate from the detention reservoir is typically controlled by a restrictor unit. Prior art restrictor units embody a flow restrictor consisting of a fixed placement flow structure such as an orifice plate, weir, gate or combination thereof. The flow restrictor is configured to release stormwater from the detention reservoir at the design release rate. Fixed placement flow restrictors are configured to discharge at the design release rate only when the detention reservoir is full. As the storage level in the detention reservoir decreases, the hydraulic head on the flow restrictor also decreases, resulting in a decrease in the release rate from the detention reservoir. Therefore, optimal use of storage volume for managing stormwater is not attained with prior art flow restrictors.
The relationship between hydraulic head and flow rate through the flow restrictor is illustrated in FIG. 1. For this illustration, the flow restrictor is a circular orifice and flow rate can be calculated with the equation: ##EQU1## Where: Q=flow rate (cubic feet/second)
D=orifice diameter (feet) PA1 H=hydraulic head (feet)
The flow rates for four different orifice diameters with hydraulic heads varying from 0.0' to 10.0' are shown in FIG. 1. The design release rate for each orifice occurs with a hydraulic head of 10.0'. For the 0.40' diameter orifice, this rate is 2.0 CFS. With the hydraulic head at 5.0', the flow rate for the 0.40' diameter orifice decreases below the design release rate to 1.4 CFS. With prior art control structures, decreases in hydraulic head on the flow restrictor reduce discharge from the detention reservoir below the design release rate resulting in ineffective use of available storage volume because more storage volume is required to detain runoff from a storm than would be required if discharge from the detention reservoir were maintained at the design release rate for all storage levels.
Ineffective use of available stormwater reservoir volume increases the costs of construction projects. In King County, Washington, for example, a developed acre of land can require up to 15,000 cubic feet of detention volume for stormwater management. Construction costs of detention reservoirs range from $5-10/cubic foot of storage. The value of the real estate occupied by the stormwater management detention reservoir is an additional cost which may be far more significant than construction costs.
Many stormwater management systems, installed in developed areas over five years ago, no longer meet current regulatory stormwater management requirements. More restrictive release rates and more stringent water quality standards have increased the storage volume requirements for detention reservoirs. Some inadequate stormwater detention systems cannot be upgraded to meet current standards, due to space or economic constraints. Stormwater detention reservoirs are typically designed to detain stormwater from the contributing watershed for a storm event of a specified return interval. A 10-year storm event is a storm of magnitude which is likely to occur once every 10 years. Larger storm events statistically occur at less frequent time intervals. Storm events of 10, 50 or 100-year return intervals are commonly used for sizing detention reservoirs. Stormwater inflow in excess of the design capacity of the detention reservoir bypasses the flow restrictor through an overflow outlet.
A flow restrictor mechanism has the potential to be blocked or clogged by debris carded in the stormwater. Blockage of the flow restrictor mechanism can cause the detention reservoir to fill to capacity and then overflow. Overflow from detention reservoirs is not uncommon and results in significantly higher flow rates than those regulated by the flow restrictor.
The design of the detention reservoir and the flow control structure is typically based on analytical methods using hydrologic models. The accuracy of the models and procedures used in the design process varies. Once in place, there is typically no convenient method for adjusting the design of currently used flow restrictors to improve the operation of the system based on actual performance.
Prior art flow restrictors are always in an open position. No flexibility exists in providing increased detention time under conditions which so permit. As a consequence, prior art flow restrictors do not allow appreciable improvements in water quality through increased detention time.
Stormwater is often routed through a vegetated swale to remove pollutants from the stormwater. Vegetated swales function primarily by slowing stormwater flow velocity with increased flow resistance from the vegetation, thereby enabling suspended solids to settle. Vegetated swales have a mixed record of success in terms of effective stormwater pollutant removal. In many instances, poor pollutant removal capabilities have been attributed to short detention time and resuspension of trapped pollutants. Pollutants such as sediment, grease, oils, nitrates, phosphates, metals, coliform bacteria and pathogens can adversely alter the physical, chemical and biological properties of the environment and decrease water quality.
Water management on agricultural and forest land is commonly practiced to improve soil moisture conditions for machine operations (such as planting and harvest) and plant growth. Controlling drainage rates from agricultural and forest land is considered to be a "Best Management Practice." Controlled drainage allows water tables to be managed more precisely, providing greater control over outflow rates and improving water quality. The benefits of controlled drainage are realized offsite through reduced peak flow rates and improved water quality. Onsite benefits include improved crop yields and improved soil operability. As with stormwater management systems, prior art fixed placement flow restrictors are typically used in agricultural and forest water management applications, often limiting the flexibility of water management alternatives.
Constructed wetlands are becoming a common means to mitigate losses of natural wetlands, manage stormwater, protect coastlines from erosion and enhance wildlife habitat. The successful design and implementation of constructed wetlands depend to a large extent on the site hydrology. With prior art fixed placement flow restrictors, maintaining desired plant species composition and wetland bathymetric conditions are often difficult tasks when constructing wetlands. It has been documented that approximately half of the constructed wetlands fail, primarily due to a lack of adequate control over the hydrology. At an average cost of $70,000/acre, construction of wetlands can be a significant project expense.
Construction and rehabilitation of tidal wetlands and marshes are increasingly more common methods of protecting coastal areas from erosion and enhancing wildlife habitat. The success of tidal wetlands in supporting a desired type of vegetation and wildlife relates to the hydroperiod and tidal fluctuations of the wetland. Plant species found in high marsh ecosystems may be desired in a specific constructed tidal marsh site, but existing tidal fluctuations or hydroperiod may prevent the establishment and success of such species.
Prior art flow control structures are limited in their effectiveness in managing stormwater and achieving other water management objectives. A system is needed which has the versatility to meet a variety of stormwater management objectives including making more effective use of available reservoir storage volume, improving water quality through increased detention time, routing stormwater, controlling wetland hydroperiod and improving agricultural and forest water management. In addition, a system is needed which allows site specific adjustment and refinement of operation.